Seed viability, germination and seedling emergence of the critically endangered stem succulent, Adenium swazicum, in South Africa

The regeneration ecology of many southern African threatened plant species is poorly understood. Temperature is considered the most important environmental factor governing seed germination. The germination temperature requirements of the critically endangered stem succulent, Adenium swazicum, were determined under controlled conditions, together with tetrazolium staining protocol tests to assess seed viability. The effects of soil medium, watering levels, depth of sowing and shading on germination/seedling emergence were also determined at the Lowveld National Botanical Garden (LNBG). Germination/seedling emergence was compared under nursery conditions within (Skukuza Nursery in Kruger National Park) and outside (LNBG) the natural distribution range of A. swazicum. The germination of viable seeds ranged 96.5-100% for a broad range of temperatures from 20 to 35 °C as well as for 30/20 °C (light/dark) alternating temperature. No germination was recorded at 5 °C and 10 °C, and relatively limited germination at 15 °C (42.9%) and 40 °C (15.4%). These results were modelled using a Generalized non-linear model, with a binomial error and a logit link function, best-fit models were second-order polynomials (i.e., quadratic). Mean Germination Time ranged from 2 to 6 days among temperature treatments. Tetrazolium together with germination results for the 20 − 35 °C temperatures showed moderate to high seed viability across populations and between years within the same population. Seedlings readily emerged irrespective of soil media (3 types) or planting depth (surface, 5 and 10 mm) but higher emergence occurred under more frequent watering. Seedlings emerged equally well under sun or shade conditions, but only subsequently survived in the shade. Despite lower temperatures recorded during October/November 2010 at LNBG (outside range: 27.2 ± 0.9 °C) compared to Skukuza (within range: 32.6 ± 0.9 °C), final germination/seedling emergence was high at both locations (LNBG: 82%; Skukuza: 94%). Together with field observations, this study shows that (a) seed germination is rapid and with high percentages after being cued by warm to hot summer temperatures, a period when rainfall is also at its highest, and (b) that seedling recruitment depends on the availability of suitably shaded microsites. This is the first study on the regeneration ecology of an Adenium sp. and will aid both in situ and ex situ conservation of A. swazicum.